Laser irradiation device

ABSTRACT

In annealing a non-single crystal silicon film through the use of a linear laser beam emitted by a YAG laser of a light source, it is the object of the present invention to prevent heterogeneity in energy caused by an optical interference produced in the linear laser beam from having an effect on the silicon film. The laser beam is divided by a mirror  604  shaped like steps into laser beams which have an optical path difference larger than the coherence length of the laser beam between them. The divided laser beams are converged on an irradiate surface  611  by the action of a cylindrical lens array  605  and a cylindrical lens  606  to homogenize the energy of the laser beam in the length direction and to determine the length of the linear laser beam. On the other hand, the laser beams divided by a cylindrical lens array  607  are converged on the irradiate surface  611  by a cylindrical lens  608  and a doublet cylindrical lens  609  to homogenize the energy in the width direction of the laser beam and to determine the width of the linear laser beam. Interference fringes parallel to the width direction of the linear laser beam disappears in the linear laser beam by the action of a mirror  604  shaped like steps. If the silicon film is annealed by the linear laser beam while the linear laser beam is being shifted in the width direction of the linear laser beam, the silicon film is remarkably homogenized as compared with a conventional silicon film.

Divisional of prior application Ser. No. 10/424,874 filed Apr. 29, 2003,now U.S. Pat. No. 7,135,390, which is a divisional application ofapplication Ser. No. 09/637,905 filed Aug. 14, 2000, now U.S. Pat. No.6,563,843

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for manufacturing asemiconductor device having a circuit constituted by a thin film and, orexample, to a device for manufacturing an electro-optical devicetypified by a liquid display device and an electric device having theelectro-optical device as a part. In this connection, in the presentspecification, a semiconductor device designates in general a devicecapable of functioning by the use of semiconductor characteristics andincludes the above electro-optical device and electric device.

2. Description of the Related Art

In recent years, research and development have been widely conducted onthe technologies for performing a laser annealing processing to anamorphous semiconductor film or a crystalline semiconductor film(semiconductor film which is not a single crystal but a polycrystal or amicro-crystal), that is, non-single crystal semiconductor film formed onan insulating substrate such as a glass substrate or the like tocrystallize the non-single crystal semiconductor film or to improve itscrystallinity. A silicon film is often used as the above semiconductorfilm.

A glass substrate has advantages that it is cheap and has goodworkability and is easy to make a large area substrate in comparisonwith a quartz substrate which has been conventionally used. This isbecause the above research and development have been carried out. Also,it is because the melting point of the glass substrate is low that alaser is widely used for crystallizing the semiconductor film. The lasercan apply high energy only to a non-single crystal film withoutincreasing the temperature of the substrate too much.

The crystalline silicon film is called a polycrystalline silicon film ora polycrystalline semiconductor film because it is made of many crystalgrains. Since the crystalline silicon film subjected to a laserannealing processing has high mobility, a thin film transistor(hereinafter referred to as TFT) is formed by the use of the crystallinesilicon film and, for example, is widely used for a monolithic liquidcrystal electro-optical device having a glass substrate and TFTs fordriving a pixel and for a driving circuit.

Also, a laser annealing method of transforming the high-power laser beamof a pulse oscillation such as an excimer laser into a square spotseveral cm square or a linear beam 10 cm or more in length at anirradiate surface by the use of an optical system and of scanning asemiconductor film with the laser beam (or moving a spot irradiated withthe laser beam relatively to an irradiate surface) has been widely usedbecause it increases mass productivity and is excellent in an industrialview point.

In particular, when a linear laser beam is used, the whole irradiatesurface is irradiated with the linear laser beam only by scanning theirradiate surface in the direction perpendicular to the direction of theline of the linear laser beam, which therefore produces high massproductivity. In contrast to this, when a spot-like laser beam is used,the irradiate surface needs to be scanned with the laser beam in theback-and-forth direction and in the right-and-left direction. Theirradiate surface is scanned with the linear laser beam in the directionperpendicular to the direction of the line of the linear laser beambecause the direction is the most efficient scanning direction. Themethod of using the linear laser beam into which the laser beam emittedfrom the excimer laser of pulse oscillation is transformed by the use ofa suitable optical system for the laser annealing processing has becomea mainstream technology.

In FIG. 1 is shown an example of the constitution of an optical systemfor transforming the cross section of the laser beam into a linear shapeat an irradiate surface. This constitution is extremely ordinary and allthe above optical systems are similar to FIG. 1. This constitution notonly transforms the cross section of the laser beam into the linearshape but also homogenizes the energy of the laser beam at the irradiatesurface. In general, an optical system homogenizing the energy of thebeam is called a beam homogenizer.

In the case where an excimer laser which is ultraviolet radiation isused as a light source, it is recommended that the base material of theabove optical system be quartz because the quartz can produce a hightransmittance. Also, it is recommended to use a coating capable ofproducing a transmittance of 99% or more to the wavelength of theexcimer laser.

First, a side view in FIG. 1 will be described. A laser beam emitted bya laser oscillator 101 is divided into the direction orthogonal to thedirection of travel of the laser beam by cylindrical lens arrays 102 aand 102 b. The applicable direction is called a vertical direction inthe present specification. When a mirror is arranged in the middle ofthe optical system, the above vertical direction is bent in thedirection of the light bent by the mirror. In this constitution, thelaser beam is divided into four portions. These divided laser beams areonce unified to one laser beam by a cylindrical lens 104. The unifiedlaser beam is reflected by a mirror 107 and then is again focused on onelaser beam at an irradiate surface 109 by a doublet cylindrical lens108. The doublet cylindrical lens means the one constituted by twocylindrical lenses. The doublet lens homogenizes the energy in the widthdirection of the linear laser beam and determines a length in the widthdirection of the laser beam.

Next, a top view will be described. The laser beam emitted by the laseroscillator 101 is divided by cylindrical lens arrays 103 into thedirection orthogonal to the direction of travel of the laser beam and inthe direction orthogonal to the vertical direction. The applicabledirection is called a lateral direction in the present specification.When a mirror is arranged in the middle of the optical system, the abovelateral direction is bent in the direction of the light bent by themirror. In this constitution, the laser beam is divided into sevenportions. These divided laser beams are once converged on one laser beamat the irradiate surface 109 by the cylindrical lens 105. Thishomogenizes the energy in the length direction of the linear laser beamand determines the length of the linear laser beam.

The above lenses are made of synthetic quartz to respond to the excimerlaser. Also, their surfaces are coated such that they well transmit theexcimer laser, whereby the transmittance of one lens to the excimerlaser is made 99% or more.

The linear laser beam transformed by the above constitution is appliedto the non-single crystal silicon film while it is gradually shifted andsuperposed in the direction of the width of the linear laser beam tosubject the whole surface of the non-single crystal silicon film tolaser annealing to thereby crystallize the non-single crystal siliconfilm or to improve the crystallinity thereof.

Next, a typical method of forming a semiconductor film to be irradiatedwith the laser beam will be described.

First, a Corning 1737 substrate 0.7 mm thick and 5 inch square wasprepared as a substrate. A SiO₂ film (silicon oxide film) having athickness of 200 nm was formed on the substrate with a plasma CVD deviceand the amorphous silicon film (hereinafter referred to as “a-Si film”)having a thickness of 50 nm was formed on the surface of the SiO₂ film.

The substrate was heated at 500° C. in a nitrogen atmosphere for 1 hourto reduce the concentration of hydrogen in the film, whereby theresistance to laser of the film was remarkably improved.

A XeCl excimer laser L3308 (wavelength=308 nm, pulse width=30 ns) madeby Ramda Corp. was used as a laser device. The laser device generates apulse oscillation laser and has a capacity producing an energy of 500mJ/pulse. The size of the laser beam is 10-30 mm (both in full width athalf maximum) at the exit of the laser beam. The exit of the laser beamis defined, in the present specification, as a plane perpendicular tothe direction of travel of the laser beam right after the laser beam isemitted by the laser irradiation device.

In general, the shape of the laser beam generated by the excimer laseris rectangular and ranges from 3 to 5 when expressed in aspect ratio,and as the position is nearer to the center of the laser beam, theintensity of the laser beam is stronger, that is, the intensity of thelaser beam shows a Gaussian distribution. The size of the above laserbeam was transformed into a linear laser beam of 125 mm×0.4 mm having auniform energy distribution by an optical system having a constitutionshown in FIG. 1.

According to the experiment of the present inventor, it was found thatthe pitch of superposition of 1/10 times the width (full width at halfmaximum) of the linear laser beam was most suitable in the case wherethe above semiconductor film was irradiated with the linear laser beam.This improved homogeneity in crystallinity in the film. In the aboveexample, the above full width at half maximum was 0.4 mm and hence thesemiconductor film was scanned and irradiated with the laser beam of theexcimer laser at a pulse frequency of 30 Hz, at a scanning speed of 1.0mm/sec, at an energy density of 420 mJ/cm² at the surface irradiatedwith the laser beam. The above-described method is an extremely ordinarymethod used for crystallizing a semiconductor film by the use of thelinear laser beam.

When the silicon film annealed with the above linear laser beam was verycarefully observed, very weak interference fringes were observed. Thisis because when the divided laser beams were again converged on oneregion, the divided laser beams interfered with each other. However,since the excimer laser has a coherence length ranging from aboutseveral micron to several tens micron, it does not produce stronginterference.

The state of the art in the excimer laser can oscillate high-power,high-repetition pulses (about 300 Hz) and hence is widely used forcrystallizing the semiconductor film. When the liquid crystal displayusing a low-temperature polysilicon TFT, which has been brought to acommercial stage in recent years, was manufactured, the excimer laser iswidely used in the crystallization process of the semiconductor film.

In recent years, the maximum power of a YAG laser has been remarkablyincreased. Since the YAG laser is a solid state laser, it is easy tohandle and maintain as compared with the excimer laser which is a gaslaser. The present inventor considered the possibility of the YAG laserbeing used for crystallizing the semiconductor film in consideration ofthe increasing power of the YAG laser.

It is well known that the YAG laser emits a laser beam having awavelength of 1065 nm as a fundamental wave. The absorption coefficientof the silicon film to the laser beam is very low and hence can not beused in this state for crystallizing the a-Si film which is one of thesilicon films. However, the laser beam can be converted into the laserbeams having shorter wavelengths by the use of the non-linear opticalcrystal. The converted laser beams are called the second harmonic (533nm), the third harmonic (355 nm), the fourth harmonic (266 nm), and thefifth harmonic (213 nm), depending on the converted wavelength.

Since the second harmonic has a wavelength of 533 nm and has asufficient absorption coefficient to the a-Si film, it can be used forcrystallizing the a-Si film. However, its absorption coefficient to thea-Si film is not so high as that of the excimer laser. The thirdharmonic, the fourth harmonic, and the fifth harmonic are very high inthe absorption coefficient to the a-Si film and hence can crystallizethe semiconductor film with a high degree of energy efficiency.

The maximum power of the third harmonic of the state-of-the-art YAGlaser is about 750 mJ/pulse. Also, the maximum power of the fourthharmonic is about 200 mJ/pulse. The maximum power of the fifth harmonicis lower than the above maximum power and hence the fifth harmonic isnot suitable for crystallizing the semiconductor film. From theviewpoint of both the power and the absorption coefficient to the a-Sifilm of the laser beam, it is best at the present time to use the secondharmonic or the third harmonic.

Next, in the case where the YAG laser is used for crystallizing thesemiconductor film, it is preferable for mass production that the shapeof the laser beam at the irradiate surface is linear. It is preferablethat the above optical system is applied to the YAG laser as it is. Thispossibility will be considered in the following.

First, the difference between the YAG laser and the excimer laser willbe described. The shape of the laser beam emitted by the excimer laseris generally rectangular and the shape of the laser beam emitted by theexcimer laser is generally circular. The dominating size of the laserbeam having large power exceeding 500 mJ/pulse and high repetition over200 Hz is about 10-30 mm and the above optical system is tailored to thesize of the laser beam. On the other hand, the size of the laser beam ofthe YAG laser over 500 mJ/pulse is a circle having a diameter of about10 mm. In order to tailor the YAG laser 10 mm in diameter to the aboveoptical system, it is recommended that the circular laser beam betransformed into an ellipsoidal one by the use of the beam expandercapable of changing the size of the laser beam. In this case, it isrecommended that the above circular laser beam be elongated by threetimes to an ellipsoidal laser beam 30 mm in long diameter and 10 mm inshort diameter by the use of the beam expander constituted bycylindrical lenses capable of elongating the size of the laser beam inone direction.

An example of an optical system in which the above beam expander isbuilt in the optical system shown in FIG. 1 to be adapted to the YAGlaser 300 will be shown in FIG. 3. FIG. 3 shows only a top view. In FIG.3 and FIG. 1, the same reference numerals designate the lenses havingthe same shape.

A cylindrical lens 301 has a focal length of 100 mm, a length and awidth of 50 mm, and a thickness of 10 mm. A laser beam enters thecylindrical lens 301. A cylindrical lens 302 has a focal length of 200mm, a length and a width of 50 mm, and a thickness of 10 mm. Theselenses are arranged at a distance of 400 mm from each other. Thiselongates the laser beam three times in one direction.

Next, the difference in coherence length between the YAG laser and theexcimer laser will be described. As described above, the excimer laserhas a coherence length of about several micron to several tens micronand hence produces a very weak optical interference when the laser beamemitted by the excimer laser passes through an optical system fordividing the laser beam and then converging it on one point. On theother hand, the YAG laser has a very long coherence length of 1 cm andhence the effect of interference produced by the YAG laser is notnegligible.

If the laser beam emitted by the YAG laser is passed through the opticalsystem shown in FIG. 3 to be transformed into a linear laser beam 200,the linear laser beam 200 has an energy distribution in which energy isrepeatedly increased or decreased like a grid pattern.

The energy distribution shaped like a grid pattern is produced by anoptical interference. In FIG. 2A, dark lines 201 designate regionshaving comparatively high energy and blank lines 202 between the darklines 201 designate regions having comparatively low energy.

If the a-Si film is crystallized with the linear laser beam 200 havingthe energy distribution shaped like a grid pattern, the a-Si film isheterogeneously crystallized. FIG. 2B shows the surface of a siliconfilm 203 crystallized with the linear laser beam. As described above,since the a-Si film is irradiated with the linear laser beam while thelinear laser beam is shifted and superposed by 1/10 times in the widthdirection of the laser beam, interference fringes parallel to the linedirection of the linear laser beam cancel each other to become light,but interference fringes 204, 205 parallel to the width direction of thelinear laser beam remain strongly dark. In FIG. 2B, dark lines 204designate regions having comparatively high energy and blank lines 205between the dark lines 204 designate regions having comparatively lowenergy.

SUMMARY OF THE INVENTION

It the object of the present invention to solve the above-mentionedproblems and to provide a laser irradiation device for producing apolycrystalline silicon film having few interference fringes.

The present inventor has invented an optical system reducing aninterference phenomenon by using the property that light beams emittedby the same light source do not interfere with each other if the lightbeams have an optical path difference of a coherence length or morebetween them. The present invention solves the above-mentioned problems,in particular, by canceling interference fringes produced in parallel tothe direction of width of the linear laser beam.

To cancel the interference fringes produced in parallel to the directionof width of the linear laser beam, it is only essential that the opticalpath difference between the laser beams divided in the lateral directionis larger than the coherence length of the laser beam emitted by a lightsource. The light source of the laser beam used in the present inventionis a YAG laser and the coherence length of the laser beam is about 1 cm.

An example of an optical system realizing the above state will be shownin FIG. 4. The big difference between the optical system shown in FIG. 4and the one shown in FIG. 3 is a reflecting mirror 401. In FIG. 4 andFIG. 3, the same reference numerals designate the lenses having the sameshape.

A mirror 401 having a reflecting surface shaped like steps is arrangedbehind cylindrical lenses 301 and 302 forming a beam expander. Themirror 401 plays a role in making laser beams having optical pathdifferences enter cylindrical lenses of a cylindrical lens array 402.For example, a laser beam entering one reflecting surface 401 a of themirror 401 changes the direction of travel and enters one cylindricallens 402 a forming the cylindrical lens array 402. Similarly, a laserbeam entering one reflecting surface 401 b other than the reflectingsurface 401 a changes the direction of travel and enters one cylindricallens 402 b forming the cylindrical lens array 402.

Since the mirror 401 is shaped like steps, the optical path length ofthe laser beam between the exit of the laser beam of the YAG laser andthe entry of the laser beam to the cylindrical lens 402 a is differentby a length d from the optical path length of the laser beam between theexit of the laser beam of the YAG laser and the entry of the laser beamto the cylindrical lens 402 b. If the length d is larger than thecoherence length of the YAG laser, the laser beam emitted from thecylindrical lens 402 a and the laser beam emitted from the cylindricallens 402 b do not interfere with each other at an irradiate surface.

The cylindrical lens array 402 similarly acts as the cylindrical lensarray 103 and divides the laser beam in the lateral direction. The laserbeam divided by the cylindrical lens array 402 is converged on anirradiate surface 404.

A constitution for dividing the laser beam in the vertical direction andthen converging it on the irradiate surface may be an optical systemsimilar to the conventional optical system shown in FIG. 1. The energydistribution of the linear laser beam produced in this way becomes adistribution having fringes parallel to the direction of length of thelaser beam 500 shown in FIG. 5A. This distribution having fringes isproduced by an optical interference. In FIG. 5A, dark lines 501designate regions having relatively high energy and blank lines 502between the dark lines 501 designate regions having relatively lowenergy.

This is the effect of the mirror 401 shaped like steps and the energydistribution having fringes parallel to the direction of width of thelinear laser beam disappears. In FIG. 5B will be shown the surface ofthe silicon film 503 crystallized with the linear laser beam. Asdescribed above, since the a-Si film is irradiated with the linear laserbeam while the linear laser beam is shifted and superposed in the widthdirection of the linear laser beam by about 1/10 times the width of theabove linear laser beam, fringes parallel to the direction of line ofthe linear laser beam cancel each other and hence are not muchconspicuous.

This can cancel fringes produced in the width direction of the linearlaser beam which might be produced when the semiconductor film isannealed with the linear laser beam of the YAG laser.

Another means for producing an optical path difference to the laser beamis a transparent plate. If the transparent plate is put before acylindrical lens forming a cylindrical lens array, it can change onlythe optical path length of the laser beam entering the cylindrical lens.However, in general, the refractive index (ranging from 1.4 to 2.5) ofthe transparent plate to the laser beam is not so large and hence, toproduce an optical path difference larger than the coherence length ofthe laser beam, the thickness of the transparent plate is required to bethree times the above coherence length.

The present invention can be applied to all laser irradiation devicesusing not only the YAG laser but also an Ar laser or the like and, inparticular, is effectively applied to the laser irradiation devicehaving a long coherence length of 0.1 mm or more. Conversely, thepresent invention does not produce a remarkable effect to the laserirradiation device having a coherence length of 0.1 mm or less.

That is, the present invention is a laser irradiation device forapplying a laser beam the cross section of which is linear at anirradiate surface, the device comprising:

a laser oscillator for outputting a laser beam;

an optical system for transforming the cross section of the laser beaminto a linear shape; and

a stage moving at least in one direction; wherein the optical systemcomprising:

an optical system 1 playing a role in dividing the laser beam in theperpendicular direction to the travel direction of the laser beam(corresponding to 607 a and 607 b in FIG. 6);

an optical system 2 playing a role in converging the divided laser beamsby the optical system 1 on an irradiate surface and in homogenizing theenergy of the laser beam in the width direction of the laser beam thecross section of which is linear at the irradiate surface (correspondingto 608 and 609 in FIG. 6);

an optical system 3 playing a role in dividing the laser beam in adirection which is included in a perpendicular face to the perpendiculardirection and in a direction perpendicular to the travel direction ofthe laser beam (corresponding to 605 in FIG. 6);

an optical system 4 playing a role in converging the divided laser beamsby the optical system 3 on an irradiate surface and in homogenizing theenergy of the laser beam in the length direction of the laser beam thecross section of which is linear at the irradiate surface (correspondingto 606 in FIG. 6); and

means for making the difference in the optical path length (which isfrom the exit of the laser beam to the irradiate surface) between thelaser beams divided by the optical system 3 larger than the coherencelength of the laser beam (corresponding to 604 in FIG. 6).

Also, another constitution of the present invention is a laserirradiation device for applying a laser beam the cross section of whichis linear at an irradiate surface, the device comprising:

a laser oscillator for outputting a laser beam;

an optical system for transforming the cross section of the laser beaminto a linear shape; and

a stage moving at least in one direction;

wherein the optical system comprises:

a cylindrical lens array 1 playing a role in dividing the laser beam inperpendicular the direction to the travel direction of the laser beam(corresponding to 607 a and 607 b in FIG. 6);

an optical system playing a role in converging the divided laser beamsby the cylindrical lens array 1 on an irradiate surface and inhomogenizing the energy of the laser beam in the width direction of thelaser beam the cross section of which is linear at the irradiate surface(corresponding to 608 and 609 in FIG. 6);

a cylindrical lens array 2 playing a role in dividing the laser beam ina direction which is included in a perpendicular face to theperpendicular to the perpendicular direction and in a directionperpendicular to the travel direction of the laser beam (correspondingto 605 in FIG. 6);

a cylindrical lens playing a role in converging the divided laser beamson by the cylindrical lens array 2 an irradiate surface and inhomogenizing the energy of the laser bean in the length direction of thelaser beam the cross section of which is linear at the irradiate surface(corresponding to 606 in FIG. 6); and

means for making the difference in the optical path length (which isfrom the exit of the laser beam to the irradiate surface) between thelaser beams divided by the cylindrical lens array 2 larger than thecoherence length of the laser beam (corresponding to 604 in FIG. 6).

In any invention described above, a mirror shaped like steps can be usedas the means described above.

Also, in any invention described above, it is desirable that thedirection of length of the laser beam the cross section of which islinear at the irradiate surface is perpendicular to the direction ofmovement of the stage moving at least in one direction, because thisimproves productivity.

Also, in any invention described above, it is desirable that theabove-mentioned laser oscillator generates the second harmonic, thethird harmonic, or the fourth harmonic of a YAG laser, because the laserirradiation device is easy to maintain and produces high productivity.

Also, in any invention described above, it is desirable that theabove-mentioned laser irradiation device further comprises a load/unloadchamber, a transfer chamber, a robot arm, a laser irradiation chamber,and a cooling chamber, because this improves productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional optical system forming a linear laserbeam;

FIG. 2A is an illustration showing a conventional energy distribution ofa linear laser beam and FIG. 2B is an illustration showing aconventional silicon film which is scanned and irradiated with a linearlaser beam in the direction perpendicular to the length direction of thelinear laser beam;

FIG. 3 is an illustration showing a conventional example of acombination of a beam expander and an optical system forming a linearlaser beam;

FIG. 4 is an illustration showing an example of a combination of a beamexpander and an optical system forming a linear laser beam which isdisclosed by the present invention;

FIG. 5A is an illustration showing the energy distribution of a linearlaser beam of the present invention and FIG. 5B is an illustrationshowing a silicon film which is scanned and irradiated with a linearlaser beam in the direction perpendicular to the length direction of thelinear laser beam of the present invention;

FIG. 6 is an illustration showing an example of a laser irradiationdevice disclosed by the present invention;

FIG. 7 is an illustration showing a part of an optical system forming alaser beam which is disclosed by the present invention;

FIG. 8 is an illustration showing lens of a part of an optical systemforming a linear laser beam of the present invention;

FIG. 9 is an illustration showing a laser irradiation device for massproduction of Embodiment 4;

FIG. 10 shows an example of a laser irradiation device of Embodiment 5;

FIG. 11A is a photograph showing a surface state of a silicon film ofEmbodiment 5 and FIG. 11B is an image of the linear laser beam taken bya CCD camera of Embodiment 5;

FIG. 12A is a photograph showing a surface state of a silicon film ofthe compared example and FIG. 12B is an image of the linear laser beamtaken by a CCD camera of the compared example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode

First, an example in which a substrate with an a-Si film 5-inch squareis irradiated with a laser beam transformed into a linear shape at anirradiate surface will be described as an object to be irradiated.

A Corning glass 1737 having a thickness of 0.7 mm is used as asubstrate. This substrate has a sufficient durability under atemperature of 600° C. A SiO₂ film is formed in 200 nm on one side ofthe substrate by a plasma CVD method. Further, an a-Si film is formed in55 nm on the SiO₂ film. Any other film forming method, for example, asputtering method may be used.

The substrate on which the SiO₂ film and the a-Si film are formed isheated in a nitrogen atmosphere at 500° C. for one hour to reduce theconcentration of hydrogen in the a-Si film. This can dramaticallyenhance the resistance of the a-Si film to a laser. The suitableconcentration of hydrogen in the a-Si film is an order of 10²⁰atoms/cm³.

A laser irradiation device will be shown in FIG. 6. The device shown inFIG. 6 is an example of a device for irradiating a substrate with alinear laser beam. A laser beam is transformed into a linear laser beamhaving a length of 125 mm and a width of 0.4 mm by an optical systemshown in FIG. 6. Since the length of the linear laser beam is 125 mm,when a substrate 5-inch square is scanned with the linear laser beam inone direction, almost whole surface of the substrate can be irradiatedwith the laser beam.

The optical system shown in FIG. 6 is an example. The linear laser beamis focused on the a-Si film. The size of the above linear laser beam isthe size of the beam when the beam is focused on the a-Si film. Theabove constitution will be described in the following.

A YAG laser oscillator 601 of pulse oscillation type oscillates a laserbeam of third harmonic (wavelength: 355 nm). The size of the above laserbeam has a diameter of 10 mm at the exit of the laser beam. The maximumpower of the laser beam is 500 mJ/pulse. The maximum repetitionfrequency is 30 Hz. A pulse width is 10 ns.

Since ultraviolet rays having a wavelength of 355 nm is used, quartzhaving a high transmittance in this wavelength region is used as thebase material of a lens. To well transmit ultraviolet rays having awavelength of 355 nm, it is recommended that the quartz lens be coatedwith a suitable material, which preferably increases energy efficiencyand can expand the life of the lens.

The circular laser beam 10 mm in diameter generated by the YAG laseroscillator 601 has the direction of travel changed 90 degrees by amirror 602. Then, the shape of the laser beam is changed into anellipsoid 30 mm in long diameter and 10 mm in short diameter by a beamexpander 603. The above beam expander 603 is constituted by cylindricallenses 301 and 302 in combination.

The laser beam transformed into an ellipsoid enters a mirror 604 shapedlike steps. Here, the laser beam enters two neighboring cylindricallenses forming a cylindrical lens array with an optical path differenced. The above optical path difference d is longer than the coherencelength of the YAG laser oscillator 601. Since the coherence length ofthe YAG laser oscillator 601 is about 1 cm, if the optical pathdifference d is longer than 1 cm, it can prevent an opticalinterference.

It is recommended that the optical path difference d be adjusted byadjusting the height of the step when the mirror 604 shaped like stepsis formed. In FIG. 7 will be shown an example of the mirror 604 shapedlike steps in which optical path difference d is 1 cm. The number ofsteps of the mirror 604 shaped like steps is five. The width of eachstep is 14 mm and the height of each step is 7 mm. When the laser beamenters the mirror 604 shaped like steps from the direction in which whenparallel light enters the above mirror 604 shaped like steps, theparallel light casts shadows 7 mm in width on the steps, the laser beamsreflected by each step enter two neighboring cylindrical lenses formingthe cylindrical lens array 605 with an optical path difference of 1 cm.

Each of laser beams reflected by the steps of the mirror 604 shaped likesteps becomes a laser beam having a width of 5 mm and enters eachcylindrical lens forming the cylindrical lens array 605. The width ofthe cylindrical lens array 605 is determined by the shape of the mirror604 shaped like steps and the width of each cylindrical lens forming thecylindrical lens array 605 becomes 15 mm in this case. When the laserbeams reflected by the mirror 604 shaped like steps are passed throughan optical system having a structure similar to FIG. 1, they become alinear laser beam at an irradiate surface. A specific example having astructure similar to FIG. 1 will be described in the following. Each ofall lenses described in the following has a curvature in the directionof width.

First, the constitution of an optical system acting in the lateraldirection will be described.

The cylindrical lens array 605 is made by setting five cylindricallenses 15 mm in width, 50 mm in length, 10 mm in thickness, and 90 mm infocal length together in array in the width direction. The cylindricallens is a flat/convex lens and a convex surface is a spherical surface.In the cylindrical lens used in the present specification, unlessotherwise specified, an incident surface is a spherical surface andother surface is a flat surface. To set the cylindrical lenses in array,it is recommended that the cylindrical lenses be bonded to each other byheating or be put in a frame and fixed- to each other from outside. Thecylindrical lens array 605 plays a role in dividing the laser beam inthe lateral direction.

The laser beams divided in the direction of the width enter acylindrical lens 606. The cylindrical lens 606 plays a role in unifyingthe laser beams divided in the width direction at an irradiate surface611. The cylindrical lens 606 has a width of 50 mm, a length of 50 mm, athickness of 5 mm, and a focal length of 2250 mm. The cylindrical lens606 homogenizes the linear laser beam in the length direction of thelinear laser beam and determines the length of the linear laser beam.The distance between the cylindrical lens array 605 and the cylindricallens 606 is 200 mm.

Next, the constitution of an optical system acting in the verticaldirection will be described.

The laser beams emitted from the cylindrical lens 606 enter acylindrical lens array 607 a at a distance of 100 mm from thecylindrical lens 606. Each cylindrical lens forming the cylindrical lensarray 607 a has a width of 3 mm, a length of 60 mm, a thickness of 3 mm,and a focal length of 300 mm. These four cylindrical lenses are puttogether in the width direction to form the cylindrical lens array 607a. The same method of making the cylindrical lens array 605 may be usedfor putting together the cylindrical lenses. The laser beam is dividedin the vertical direction by the cylindrical lens array 607 a.

The laser beams emitted from the cylindrical lens 607 a enter acylindrical lens array 607 b at a distance of 443 mm from thecylindrical lens 607 a. Each cylindrical lens forming the cylindricallens array 607 b has a width of 3 mm, a length of 60 mm, a thickness of3 mm, and a focal length of 450 mm. These four cylindrical lenses areput together in the width direction to form the cylindrical lens array607 b. The same method of making the cylindrical lens array 605 may beused for putting together the cylindrical lenses. The laser beamsdivided by the cylindrical lens array 607 a enter each cylindrical lensforming the cylindrical lens array 607 b.

The laser beams emitted from the cylindrical lens array 607 b enter acylindrical lens 608 at a distance of 89 mm from the cylindrical lensarray 607 b. The cylindrical lens 608 has a width of 50 mm, a length of60 mm, a thickness of 5 mm, and a focal length of 350 mm. The laserbeams enter the flat surface of the cylindrical lens 608 havingflat/convex surfaces. The laser beams are once unified by thecylindrical lens 608 on the same surface. The same surface is at thefocus of the cylindrical lens 608. Since the same surface is in themiddle of an optical path, the unified laser beams are separated again.

The laser beams emitted from the cylindrical lens 608 enter a doubletcylindrical lens 609 at a distance of 1377 mm from the cylindrical lens608. A mirror 613 is interposed between the cylindrical lens 608 and thedoublet cylindrical lens 609 because of the arrangement of lenses tochange the direction of travel of the laser beam in the downwarddirection, whereby the substrate can be horizontally arranged which isan object to be irradiated with the laser beam.

The doublet cylindrical lens 609 has a width of 70 mm, a length of 140mm, a thickness of 31 mm, and a focal length of 177 mm. The laser beamsdivided in the vertical direction are unified at the irradiate surface611 by the doublet cylindrical lens 609. The doublet cylindrical lens609 homogenizes the linear laser beam in the width direction anddetermines the width of the linear laser beam.

A quartz window 610 having a thickness of 15 mm is arranged between thedoublet cylindrical lens 609 and the irradiate surface 611. The distancebetween the doublet cylindrical lens 609 and the quartz window 610 is 70mm and the distance between the quartz window 610 and the irradiatesurface 611 is 140 mm.

The quartz window 610 is a window fixed to a chamber 612 separating thesubstrate with a semiconductor film formed thereon from outside air topass the laser beams. The chamber 612 has an exhaust unit and a gas line(both not shown) connected thereto and the atmosphere in the chamber 612can be suitably adjusted.

An example of a specification of the doublet cylindrical lens 609 willbe described with reference to FIG. 8. The doublet cylindrical lens 609has a width of 70 mm, a length of 140 mm, a thickness of 31 mm, and afocal length of 177 mm. A first cylindrical lens has surfaces 801 and802 and a thickness of 10 mm. The radius of curvature of the surface 801entered by the laser beam is 125 mm and the radius of curvature of theother surface 802 is 69 mm. A second cylindrical lens has surfaces 803and 804 and a thickness of 20 mm. The radius of curvature of the surface803 entered by the laser beam is 75 mm and the radius of curvature ofthe other surface 802 is −226 mm. The sign attached to the radius ofcurvature designates the direction of the curvature. Also, the secondcylindrical lens is arranged such that the surface 803 entered by thelaser beam is at a distance of 1 mm from the surface 802 of the firstcylindrical lens. That is, the thickness (31 mm) of the doubletcylindrical lens 609 is the sum of the thickness (10 mm) of the firstcylindrical lens, the thickness (20 mm) of the second cylindrical lens,and the distance (1 mm) between the first cylindrical lens and thesecond cylindrical lens.

To protect the optical system, the atmosphere around the optical systemmay be a gas resisting to react with a lens-coating material such as anitrogen gas. For this reason, the optical system may be put in anoptical system protection chamber. It is recommended that coated quartzbe used for a window through which the laser beams enter or exit fromthe optical system protection chamber because the coated quartz has atransmittance of 99% or more.

If the energy distribution in the line direction of the linear laserbeam is made within ±5%, the linear laser beam can crystallize the a-Sifilm homogeneously. Preferably, the energy distribution in the linedirection of the linear laser beam is made within ±3% and, morepreferably, within ±1% to more homogeneously crystallize the a-Si film.To make the energy distribution uniform, the lenses need to be alignedwith high accuracy.

The substrate having the a-Si film is arranged at the irradiate surface611 and a stage 614 is moved at a constant speed in the directionperpendicular to the length direction of the linear laser beam (in thedirection of an arrow in FIG. 6) by the use of a moving mechanism 615with the substrate being irradiated with the laser beam, whereby thewhole surface of the substrate can be irradiated with the laser beam. Aball-screw type mechanism or a linear motor can be used as the movingmechanism 615.

It is recommended that irradiation conditions be determined according tothe following guidelines: energy density of linear laser beam=50-500mJ/cm²; moving speed of stage=0.1-2 mm/sec; and oscillation frequency oflaser oscillator=30 Hz.

The above conditions may be varied according to the pulse width of thelaser oscillator, the state of the semiconductor film, and thespecification of a device to be manufactured. Accordingly, theconditions need to be finely set by a practicing person.

The atmosphere in the chamber 612 when the substrate is irradiated withthe laser beam is the air set at 20° C. and may be replaced by a H₂ gas.The atmosphere is replaced mainly to prevent the contamination of thesubstrate. The gas is supplied through a gas cylinder. The aboveatmosphere may be H₂, He, N₂, Ar, or a mixed gas of these gases. Also,the evacuation of the chamber to a vacuum (10⁻¹ Torr or less) alsoproduces an effect of preventing the contamination of the substrate.

In addition to the Corning glass 1737, a glass substrate such as Corningglass 7059, AN 100 can be used as a substrate, or a quartz substrate maybe used.

If the spot of the substrate which is being irradiated with the laserbeam is further irradiated and heated with strong light, for example,with an infrared lamp, it is possible to reduce the energy of the laserbeam as compared with the case where the substrate is not heated withthe strong light. The substrate may be heated with a heater arrangedunder the substrate. In the case where the linear laser beam is furtherelongated and is applied to a substrate having a larger area or in thecase where the energy of the laser beam is not sufficient, the supply ofenergy by heating is effective.

The laser irradiation device in accordance with the present inventioncan be applied not only to a non-single crystal silicon film but also tothe other non-single crystal semiconductor films, for example, asemiconductor containing germanium and other non-single crystalsemiconductor films.

It is recommended that a semiconductor device, for example, a liquidcrystal display made of low-temperature polysilicon TFTs, or asemiconductor device invented by a practicing person be manufactured byusing a semiconductor film crystallized with the above laser irradiationdevice by a publicly known method.

Embodiment 1

In the present embodiment, an example will be described in which apolycrystalline silicon film is irradiated with a laser beam. The laserirradiation device described in the above preferred embodiment is usedas a laser irradiation device for the present embodiment mode.

A Corning glass 1737 having a thickness of 0.7 mm is used as asubstrate. The substrate has sufficient durability if it is used under600° C. An SiO₂ film is formed in 200 nm on one surface of the substrateby a plasma CVD method. Further, an a-Si film is formed in 55 nm on theSiO₂ film. Any other film forming method, for example, a sputteringmethod may be used.

Next, the above-mentioned a-Si film is crystallized by the methoddisclosed in Japanese Patent Laid-Open No. 7-130652. The method will bedescribed briefly in the following. The above a-Si film is coated with anickel acetate water solution having a concentration of 10 ppm and thenis heated in a nitrogen atmosphere at 550° C. for 4 hours, whereby thea-Si film is crystallized. It is recommended that a spin coat method,for example, be used for applying the nickel acetate water solution. Thea-Si film to which nickel is added is crystallized in a short period atlow temperatures. It is thought that this is because the nickel acts asthe seed crystal of crystal growth to facilitate the crystal growth.

If the polycrystalline silicon film crystallized by the above method isirradiated with the laser beam, it has higher characteristics as amaterial of a semiconductor device. Accordingly, to improve thecharacteristics of the above polycrystalline silicon film, the abovepolycrystalline silicon film is irradiated with the laser beam by usingthe laser irradiation device used in the preferred embodiment of thepresent invention.

It is recommended that a semiconductor device, for example, a liquidcrystal display made of low-temperature polysilicon TFTs, or asemiconductor device invented by a practicing person be manufactured byusing a semiconductor film crystallized with the above laser irradiationdevice by a publicly known method. The preferred embodiment mode of thepresent invention and the embodiment 1 can be used in combination.

Embodiment 2

In the present embodiment, an example will be described in which thesecond harmonic of a YAG laser is used as a laser oscillator. Theadvantage of using the second harmonic resides in that an optical lensresists being degraded by the second harmonic. Also, since the secondharmonic has a lower reflectance to the a-Si film than the thirdharmonic or the fourth harmonic, the energy efficiency obtained when thesecond harmonic is used is slightly lower than the energy efficiencyobtained when the third harmonic or the fourth harmonic is used. Themaximum pulse energy now in existence is 1400 mJ/pulse. This is twotimes the third harmonic and hence it is recommended that the elongatedlinear laser beam of the second harmonic be used for crystallizing thea-Si film formed on a substrate having a large area.

It is recommended that the same laser irradiation device and method asthose described in the preferred embodiment of the present invention beused as the device and method for transforming the second harmonic intoa linear laser beam and applying it to semiconductor film. However,since the third harmonic is different in a wavelength from the secondharmonic, it is necessary to change the focal point. In the case of thepresent embodiment, it is recommended that the distance between thequartz window 610 and the irradiate surface 611 be changed to 150 mm.The coating of the lens is adapted to the wavelength (530 nm) of thesecond harmonic of the YAG laser.

The embodiment 2 can be combined with the embodiment 1.

Embodiment 3

In the present embodiment, an example will be described in which thefourth harmonic of a YAG laser is used as a laser oscillator. Theadvantage of using the fourth harmonic resides in that the absorptioncoefficient of the fourth harmonic to a silicon film is very high.

It is recommended that the same laser irradiation device and method asthose described in the preferred embodiment of the present invention beused as the device and method for transforming the fourth harmonic intoa linear laser beam and applying it to semiconductor film. However,since the fourth harmonic is different in a wavelength from the thirdharmonic, it is necessary to change the focal point. In the case of thepresent embodiment, it is recommended that the distance between thequartz window 610 and the irradiate surface 611 be changed to 126 mm.The coating of the lens is adapted to the wavelength (266 nm) of thefourth harmonic of the YAG laser.

The embodiment 3 can be combined with the embodiment 1.

Embodiment 4

In the present embodiment, an example of a laser irradiation device formass production will be described with reference to FIG. 9. FIG. 9 is atop view of a laser irradiation device.

A substrate is carried from a load/unload chamber 901 by the use of acarrying robot arm 903 mounted in a transfer chamber 902. First, thesubstrate is aligned in an alignment chamber 904 and then is carried toa pre-heat chamber 905. In the pre-heat chamber 905, the substrate ispreviously heated to a desired temperature of about 300° C., forexample, by the use of an infrared lamp heater. Then, the substrate isplaced in a laser irradiation chamber 907 via a gate valve 906 and thenthe gate valve 906 is closed.

A laser beam is emitted by the laser oscillator 900 described in thepreferred embodiment mode in accordance with the present invention andthen is bent downward. 90 degrees by a mirror (not shown) placeddirectly above a quartz window 910 via an optical system 909 and istransformed into a linear laser beam at an irradiate surface in thelaser irradiation chamber 907 via the quartz window 910. The laser beamis applied to the substrate placed at the irradiate surface. It isrecommended that the above-mentioned optical system be used as theoptical system 909, or the one similar to the optical system may beused.

The laser irradiation chamber 907 is evacuated by a vacuum pump 911 tomake the atmosphere of the chamber 907 a high vacuum of about 10⁻³ Pabefore the irradiation of the laser beam, or the atmosphere of the laserirradiation chamber 907 is made a desired atmosphere by the vacuum pump911 and a gas cylinder 912. As described above, the above atmosphere maybe He, Ar, H₂, or the mixed gas of them.

Then, the substrate is scanned and irradiated with the linear laser beammoved by a moving mechanism 913. At this time, an infrared lamp (notshown) may be applied to the spot of the substrate irradiated with thelinear laser beam.

After the end of the irradiation of the laser beam, the substrate iscarried to a cooling chamber 908 to be allowed to cool slowly and thenis returned to the load/unload chamber 901 via the alignment chamber904. In this manner, many substrates can be annealed with laser byrepeating these actions.

The embodiment 4 can be used in combination with the preferredembodiment mode and the other embodiments of the present invention.

Embodiment 5

In Embodiment 5, an example that a quartz plate is used as a means forproducing an optical path difference is described, as shown in FIG. 10.This quartz plate is transparent with respect to the laser beam.

In FIG. 10, a structural example of an optical system for processing asectional shape of the laser beam into a linear shape on an irradiatesurface is shown. FIG. 10 is a drawing of a top view.

A part of a laser barn emitted by laser oscillator 1001 enters a quartzplate 100 with a thickness of 15 mm, so that an optical path differenceis generated between the part and another part of the laser beam thatdoes not enter the quartz plate 1000. A refractive index of the quartzplate 1000 is about 1.5 with respect to wave length 532 nm. Accordingly,an optical difference of about 7 mm is generated. Such the opticaldifference of 7 mm is equal to the coherence length of the YAG laser, aneffect to prevent an interference is expected.

The laser beam which is lengthened through the quartz plate 1000 entersone cylindrical lens of a cylindrical lens array 1003, while the laserbeam which does not pass through the quartz plate 1000 enters the othercylindrical lens of the cylindrical lens array 1003, to be divided intotwo portions. Then, the laser beams are unified to one laser beam on anirradiate surface 1009 by a cylindrical lens 1005. Thus, the energy inthe length direction of the linear laser beam is homogenized and thelength of the linear laser beam is determined.

Like this, the laser beam having the optical path difference by thequartz plate is irradiated into an amorphous silicon (a-Si) film. Asshown in FIG. 11A, fringes in a direction parallel to the widthdirection of the linear laser beam can be eliminated. It is noted that avertical direction of FIG. 11A corresponds to the width direction of thelinear laser beam.

FIG. 11A is a photograph showing a surface state of an amorphous siliconfilm irradiated with the linear laser beam processed through the presentembodiment. Further, FIG. 11B is an image of the linear laser beamprocessed through the present embodiment taken by a CCD camera.

In order to compare, without the optical path difference, that is, alinear laser beam processed without using the quartz plate 1000 isirradiated into an amorphous silicon film. A surface state of such theamorphous silicon film is shown in a photograph of FIG. 12A. Inaddition, FIG. 12B is an image of the linear laser beam without theoptical path difference, that is, which is processed without using thequartz plate 1000 is taken by CCD camera. Because of the interference,the fringes parallel to the width direction of the linear laser beam aregenerated.

In Embodiment 5, an example that the laser beam is divided into twoportions and unified is indicated. However, in a case that a laser beamcan be divided into three portions or more and unified into one, if thedivided laser beams have efficient optical path differences, a sameeffect as the present embodiment can be obtained. For example, when thelaser beam is divided into three portions, it is proper to unify onelaser beam not entering the quartz plate, another laser beam passingthrough a quartz plate with a thickness t, and another laser beampassing through a quartz plate with a thickness 2t. Further, thethickness t can be determined taking account of coherence length of thelaser beam used.

Furthermore, reference numeral 1007 indicates a mirror in FIG. 10.

The present invention can extremely decrease strong and weak portions ofinterference fringes generated parallel to the width direction of thelinear laser beam.

1. A method of fabricating a semiconductor device, the methodcomprising: irradiating a beam expander with a laser beam from a laseroscillator; dividing the laser beam passed through the beam expanderinto at least first and second laser beams having a difference in anoptical path length larger than a coherence length of the laser beam ina longitudinal direction; overlapping the first and second laser beamsto homogenize the first and second laser beams at an irradiation surfaceof a semiconductor film; converging the first and second laser beams ina width direction of the first and second laser beams to irradiate thefirst and second laser beams at the irradiation surface, the first andsecond laser beams having a linear shape in a cross section at theirradiation surface; and crystallizing the semiconductor film by theirradiation of the converged first and second laser beams.
 2. The methodof fabricating a semiconductor device according to claim 1, wherein thelaser beam passed through the beam expander is divided by a step-likemirror.
 3. The method of fabricating a semiconductor device according toclaim 1, wherein the laser oscillator generates a second harmonic of aYAG laser.
 4. The method of fabricating a semiconductor device accordingto claim 1, wherein the laser oscillator generates a third harmonic of aYAG laser.
 5. The method of fabricating a semiconductor device accordingto claim 1, wherein the laser oscillator generates a fourth harmonic ofa YAG laser.
 6. The method of fabricating a semiconductor deviceaccording to claim 1, wherein the laser oscillator generates an Arlaser.
 7. A method of fabricating a semiconductor device, the methodcomprising: irradiating first and second cylindrical lenses with a laserbeam from a laser oscillator; dividing the laser beam transformed by thefirst and second cylindrical lenses into at least first and second laserbeams having a difference in an optical path length larger than acoherence length of the laser beam in a longitudinal direction;overlapping the first and second laser beams to homogenize the first andsecond laser beams at an irradiation surface of a semiconductor film;converging the first and second laser beams in a width direction of thefirst and second laser beams to irradiate the first and second laserbeams at the irradiation surface, the first and second laser beamshaving a linear shape in a cross section at the irradiation surface; andcrystallizing the semiconductor film by the irradiation of the convergedfirst and second laser beams.
 8. The method of fabricating asemiconductor device according to claim 7, wherein the laser beamtransformed by the first and second cylindrical lenses is divided by astep-like mirror.
 9. The method of fabricating a semiconductor deviceaccording to claim 7, wherein the laser oscillator generates a secondharmonic of a YAG laser.
 10. The method of fabricating a semiconductordevice according to claim 7, wherein the laser oscillator generates athird harmonic of a YAG laser.
 11. The method of fabricating asemiconductor device according to claim 7, wherein the laser oscillatorgenerates a fourth harmonic of a YAG laser.
 12. The method offabricating a semiconductor device according to claim 7, wherein thelaser oscillator generates an Ar laser.